1. Field of the Invention
The invention relates to high frequency switching power supplies, and more particularly to a hysteretic regulator adapted for maintaining a constant switching frequency in a switching power supply.
2. Background Art
High frequency switching power supplies are controlled or regulated by several different control schemes. The advantages of some methods are the disadvantages of others, and vice versa. To date no control scheme retains all the advantages of all the popular methods, while eliminating the disadvantages, of these methods. Desirable attributes of a switching regulator include immediate response, inherent power supply stability, and fixed frequency. There is a need for an improved regulator providing more of these attributes.
Hysteretic Regulator—Historically, the earliest control method for a switching power supply was known as the hysteretic regulator, wherein the output signal was fed back to the inputs of a window comparator, with upper and lower limits Vmax and Vmin, respectively. In the steady state, the feedback signal is maintained within the upper and lower bounds of the window. Other names for this control method are the “Constant Ripple Regulator”, since the output ripple is constant and equal to Vmax−Vmin, and the “Bang-Bang” regulator, since the regulator reacts to the feedback signal by controlling the main switch on or off, causing the output to bounce around between the window limits. A desirable feature of the hysteretic regulator is that it is fast responding and inherently stable, requiring no compensation of the feedback signal. Curing stability problems can be quite challenging, and the inability to solve the compensation network correctly has caused many otherwise good designs to become unworkable.
However, due to the nature of how output voltage ripple is determined and to the dependence of this ripple on the relative magnitudes of the input and output voltages, the frequency of operation of the hysteretic regulator must change with input voltage. This is the most significant drawback of the hysteretic regulator, since input filtering must be determined based on the lowest frequency of operation, corresponding to the lowest Vin. The input filter requires physically larger components with decreasing Vin, and the filter becomes wasted space and cost at higher Vin. Some claim to have observed circuit subharmonic oscillations with hysteretic regulators, causing noise and interference with sensitive control signals.
The operation of the standard hysteretic regulator can be explained by the ideal waveforms in FIG. 1. Shown are the ripple voltages for two different values of Vin and a set value of Vout for a buck regulator, which is a typical application for this regulator. (The invention described herein applies to any switching regulator; the buck regulator is chosen as an example.) In FIG. 1, Vmax−Vmin is fixed, and f is variable. The nominal value of the output voltage is a line that is midway between the upper and lower bounds of each window and is labeled Vnom.
The time it takes the lower waveform to traverse upward through the window is shorter than that of the upper waveform because the input voltage in the upper waveform is lower. The slope of this part of the waveform is to the first order proportional to the difference between the input and output voltages. In both cases the downward slope of both waveforms is the same, since the slope during this part of the waveform is independent of Vin, and Vout is the same in both cases. With a set Vmax−Vmin, the time it takes to traverse the window in the downward direction is therefore the same in both cases. The difference in the upwards transition time results in different periods Ts1 and Ts2, and different switching frequencies, dependent upon Vin. Typically, switching frequencies can vary by ratios of between 6:1 and 10:1.
PWM Regulator—The most common control method used in today's high frequency switching power supplies is pulse width modulation, wherein the pulse width of the switch connecting the input to the output is varied (modulated) to provide the desired output voltage. Operation of the typical pulse width modulator, or PWM, is shown in FIG. 2, again for a buck regulator as an example, where the period T starts at time t0, f is fixed, and pulse width is variable. As Vin changes, the falling edge (or leading edge, in a leading edge blanking system) of the pulse width moves, regulating the output voltage to the desired amount. The arrows in FIG. 2 on the falling edges of the pulses signify the moving edges of the pulses. With Vin stable, this edge is automatically adjusted to compensate for changes in temperature and conduction losses due to load. The input voltage of the upper waveform in FIG. 2 is less than the input voltage of the lower waveform, since it takes a longer pulse width (and thus more time for energy transfer) to regulate the output. The period is the same for all cases of Vin and load variation in a PWM.
Although fixed frequency is a desirable characteristic in a regulator, the PWM has its disadvantages. Most importantly, the response time of the control circuit is compromised in that it must be slowed down to prevent circuit instabilities. As mentioned above, the required circuitry can be challenging. Without slowing down the response, wild oscillations of the output voltage can result, and the output may never become stable. The technique used to correct this problem is to provide feedback compensation, and a typical compensation network is provided in the feedback network around the error amplifier, as shown in FIG. 3. The output Verror of this function block will amplify and delay signals from the feedback signal Vfb due to the resistors and capacitors in the feedback path. This intentional delay is inserted to provide stability to the system.
The PWM's advantage is fixed frequency, which is not achievable with the basic hysteretic regulator. The hysteretic regulator's advantage is fast response and inherent stability, which is not possible in the PWM without compromise. What is desired is a regulator that retains the advantages of both regulators, and which will by default eliminate the disadvantages of both regulators. In summary, the characteristics desired in this regulator are fast response (hysteretic regulator), inherent electrical stability (hysteretic regulator), and fixed frequency (PWM).
Several modifications to the hysteretic regulator have been made by others attempting to obtain these characteristics.
It has been attempted to keep the frequency constant in a hysteretic regulator by adjusting the window. One method involves using feedforward of the input voltage. This keeps f relatively constant, but suffers from nonlinearity over the range of Vin, and the input filter must be determined using the worst case operating condition.
Another method keeps f constant over the midrange of duty cycle D, but the frequency changes significantly outside this range. Further, the range of variable frequency is typically in the operating region of today's buck regulators for many significant high volume applications. This method is used in the Texas Instruments TPS 5211 Hysteretic Regulator Controller, described at http://www-s.ti.com/sc/ds/tps5211.pdf.
In another method, used in the Analog Devices ADP 3205 Core Controller, output ripple is dependent upon the ESR of the output capacitor and the absolute value of the output inductor. Both of these quantities change with temperature, and the output capacitor ESR can change with age, depending upon the type of capacitor used. In addition, the capacitor ESR changes with operating frequency. Analog Devices has a method of correcting for variances in ESR, but this method does not compensate for changes in Vin. See the description of the ADP3205, especially FIG. 4, at http://www.anatog.com/productSelection/pdf/ADP3205_prj.pdf.